The present invention relates to a brushless motor which is used as a capstan motor for VCRs and the like. More specifically, the present invention relates to a brushless motor driven by a PWM (Pulse Width Modulation) method.
As a capstan motor for VCR, a brushless motor comprising the following is adapted as shown in FIG. 1. Rotor 5 which rotates together with rotational shaft 10 is held by bearing 20. Stator 30 has stator core 32 with drive coil 31 wound therearound. A motor drive circuit 60 is formed on metal substrate 40 and which has a drive IC. Brushless motor 1 employs the direct PWM method in which electric flow to drive coil 31 is controlled by turning on or off a power transistor (a switching component) of motor drive circuit 60 and modulating the width of a switching pulse to the switching component.
According to the PWM method, the amount of electricity wasted as heat generated in a conventional motor drive circuit can be dramatically decreased. In addition, energy is efficiently saved while driving the motor. Further, few new parts are needed when the method is adapted. Therefore, the method provides high cost-performance.
FIGS. 2(A), (B), 3 and 4 describe the PWM method. FIG. 2(A) shows a motor drive circuit of a brushless motor employing the direct PWM method supplying electric power from a source to a motor drive coil. FIG. 2(B) shows the same when regenerative current is caused due to counterelectromotive force generated in the drive coil as the supply of the electric power from the source to the drive coil is suspended. FIG. 3 shows waveforms of voltage and current applied to the drive coil during one phase when the control operations as described in FIGS. 2(A) and (B) are performed. FIG. 4 shows waveforms of voltage and current for the following periods within period (a) in FIG. 3: period (b) in which voltage is applied to the drive coil; and period (c) in which application of voltage to the drive coil is suspended.
As shown in FIGS. 2(A) and 3, when power transistor Q4 and power transistor Q1 are on, voltage VM is applied from source 66 to drive coil 31 such that current flows in drive coil 31. This current flows to ground M-GND of motor source 66 via power transistor Q4 (during period b in FIG. 4). The motor current gradually increases, corresponding to the time constant of drive coil 31, as shown in FIG. 4.
On the other hand, as shown in FIGS. 2(B) and 3, when power transistor Q1 is turned off while power transistor Q4 is still on, application of voltage VM from motor source 66 to drive coil 31 is interrupted. However, counterelectromotive forces E1 and E2 are generated in each drive coil 31. Hence, regenerative current flows in drive coil 31 via diode 61 as motor current. The regenerative current gradually decreases corresponding to the time constant of drive coil 31, as shown in FIG. 4. Before the regenerative current reaches the minimum value, power transistor Q1 is turned on such that current is supplied from motor source 66.
As described above, a part of the motor current is supplied by the regenerative current in brushless motor 1. Therefore, the amount of current (electricity) supplied from the outside can be reduced. Also, the power transistors through which the motor current flows are constantly saturated; hence the amount of electricity consumed in motor drive circuit 60 can be minimized.
However, in brushless motor 1 employing the direct PWM method, the voltage applied to drive coil 31 fluctuates between drive source voltage VM and ground potential M-GND in a short period of time. As a result, voltage applied to wiring between motor drive circuit 60 and coil 31 and voltage applied to coil 31 shows rapid fluctuations, causing electromagnetic noise which has various negative effects on the operation of apparatus having the motor. This electromagnetic noise is alleviated by motor parts, which form a capacitive coupling with drive coil 31 and the wiring, e.g. stator core 32, around which drive coil 31 is wound, or an iron plate sandwiching an insulating layer with a wiring on metal substrate 40 as a circuit substrate of the motor, to help the electromagnetic noise to diffuse.
Further, the current from motor source 66 is supplied during only period b in FIG. 4. It is suspended during period c which follows period b. Therefore, pulse current, which can be turned on or off with a PWM carrier frequency, flows through the wiring of motor source 66 on metal substrate 40. This pulse current generates the electromagnetic noise to which is propagated by the stator core 32 and metal substrate 40 The pulse current also causes undesirable results in the operation of the apparatus by generating ripples in motor source 66.
Considering the above issues, the present invention intends to provide a brushless motor employing the PWM method which has a configuration to suppress generation of electromagnetic noise.
To accomplish the above purpose, the present invention provides a brushless motor employing the direct PWM method which controls an electric flow to a drive coil by modulating the width of a switching pulse to a switching component wherein at least one of a metal plate forming a metal substrate and a stator core is short-circuited to a fixed electric potential. Also, an insulating resistance between a mounting portion of the motor to be connected to a chassis of a main body and the metal plate and an insulating resistance between the mounting portion and the stator core are established to be higher than 1K ohm.
According to the present invention, when a brushless motor is driven by the direct PWM method, both voltage applied to the wiring between a motor drive circuit and a drive coil and voltage applied to the drive coil itself show rapid fluctuations. The electric potentials of a metal plate as a base of a metal substrate and a stator core, which form capacitive coupling with the above parts are fixed. Hence, propagation of electromagnetic noise by those parts can be prevented. In the present invention, the fixed electric potential can be a ground potential of the motor drive circuit and the source potential.
It is preferable that a capacitor with a capacitance of 0.1 micro fared or higher, is electrically connected to the motor source in parallel at a position close to the motor drive circuit according to the present invention. In this configuration, even when pulse current flows through the wiring of the motor source, ripples in the motor source can be absorbed by the capacitor. This ensures that the apparatus can function optimally.
According to the present invention, a mounting portion is a bearing holder which is made of a conductive resin and which holds a bearing, for example. In the case that a rotor comprises a pulley, electrostatic potential may be generated due to the movement of the pulley and a connecting belt. Since the bearing holder is formed of a conductive resin, the electrostatic potential built up in the rotor including the pulley can be removed. Also, even when the stator core is held at the ground electric potential, no negative effects are seen in the operation of the apparatus due to the fact that the bearing holder is made of a conductive resin. Even though the bearing holder is fixed to a chassis of a main body of the apparatus, the chassis will not be short-circuited with ground electric potential M-GND.
In the present invention, the mounting portion can be a metal bearing holder holding a bearing. In this case, it is preferable that insulators are placed between the bearing holder and the metal plate and between the bearing holder and the stator core.
In the above case, the bearing holder has a hole, into which a member with a screw hole, made of a conductive resin, is press-fitted. It is preferable that the bearing holder and the chassis of the main body are fixed with a screw screwed in the hole. In this configuration, the bearing holder is made of a metal, and [the motor] is connected with the chassis via the member with a screw hole made of a conductive resin. Therefore, even when a pulley is formed on the rotor, electrostatic potential pooled in the rotor can be removed.